The invention relates to a clock generator for an imaging device and a method of generating a clock signal for the imaging operation.
When setting an image on a printing form, which is generally clamped on a cylinder, the desired information is transferred to the printing form with the aid of an imaging or image-setting power source. With the imaging power source, the entire area of the printing form whereon an image is to be set is scanned successively or at least to some extent in parallel, if the imaging power source is formed of a number of individual power sources. In this regard, the information is transferred to the printing form in accordance with an original in the form of individual pixels or dots or groups thereof.
In order to achieve high-quality imaging on the printing form, on the one hand, the imaging power source must have adequate focussing properties or the imaging device must have appropriate elements with the aid of which ultimately adequate focussing can be achieved. On the other hand, it is necessary for the pixels or dots, respectively, to be written into the printing form exactly at the assigned position thereof. This places high requirements on the positioning of the imaging power source relative to the printing form, in particular, in the case of high-resolution imaging. The positioning can be carried out by mutually coordinated movements of the printing form and the imaging light source. In this regard, the printing form is generally subjected to a rotational movement and the imaging power source is subjected to a translational movement parallel to the longitudinal axis of the cylinder whereon the printing form is clamped.
An incremental encoder can be used for registering the rotational movement of the printing form. The resolution of conventional incremental encoders is generally lower, however, than the positioning accuracy required in the circumferential direction of the printing form. Therefore, various devices for increasing the resolution in imaging devices and in other devices, respectively, have already been proposed heretofore.
U.S. Pat. No. 5,174,205 discloses a control system for a discharge device for setting an image on a printing form which is clamped on a cylinder. Arranged on the cylinder is a rotary encoder which emits a signal dependent upon the rotational angle of the cylinder. The signal from the rotary encoder is fed to a phase locked loop-circuit, which generates therefrom a signal with a higher resolution, which is synchronized with the original angle signal. The discharge device is controlled based upon the signal generated in this manner, correction data, in particular, for correcting geometric errors of the printing form, being also taken into consideration.
The published German Patent Document DE 42 26 236 C1 describes a device for controlling electronically triggerable devices which are arranged on a rectilinearly reciprocatable carriage, slide or the like. The control device comprises an incremental encoder which outputs a clock signal as a function of the position of the carriage, slide or the like, and a fine-step timing generator which generates a multiplicity of fine-step clock cycles between successive increments of the incremental encoder. A triggering cycle for the electronically triggerable devices, respectively, is generated after a prescribed or predefined number of fine-step clock cycles. In order also to achieve highly accurate triggering of the electronic devices during nonuniform movement of the carriage, slide or the like, the fine-step timing generator is coordinated with the incremental encoder. For this purpose, a desired or nominal value for the number of fine-step clock cycles within the time period between two increments of the incremental encoder is prescribed or predefined. If the incremental encoder outputs a further clock signal before the nominal value has been reached, the fine-step timing generator then generates the fine-step clock cycles missing from the nominal value as quickly as possible, i.e., at the system clock rate. If, on the other hand, the nominal value has been reached before the incremental encoder outputs a further clock signal, the generation of further fine-step clock cycles is then stopped until the further clock signal has been outputted by the incremental encoder.
U.S. Pat. No. 6,057,715 describes a clock generator for generating a clock signal of any desired frequency from a reference clock signal. The clock generator has a counter which in each case counts up at the cycle rate of the reference clock signal and outputs appropriate numeric values. Through the intermediary of a sine-wave table, the clock generator converts successive numeric values into function values of a prescribed or predefined number of sine waves. From the function values, with the aid of a digital/analog converter, an analog sinusoidal signal is generated, which is filtered and fed to a comparator. The comparator converts the sinusoidal signal, by comparison with a prescribed threshold, into a clock signal of square-wave form. The frequency of the thus generated clock signal, in relation to the frequency of the reference clock signal, is determined by the ratio between the number of sine waves which are stored in the sine-wave table as function values for one pass through the counter, and the size of the counter.
The conventional devices, respectively, have the disadvantage that fluctuations in the rotational speed are not or are only inadequately taken into consideration. These fluctuations in the rotational speed therefore lead to a reduction in the achievable accuracy when the clock signals generated by the conventional devices are used for imaging on printing forms.
Furthermore, the published German Patent Document DE 27 29 697 A1 describes a method for interpolating signals from a sine-cosine rotary encoder. The method is based upon the fact that the sine and cosine signals from the rotary encoder, after being digitized by an A/D (analog/digital) transducer or converter, are subjected to the arctan formation in order to calculate the phase angle. The published European Patent Document EP 0 484 576 B1 discloses a device for subdividing analog periodic signals. The interpolation described in this document is based upon a method wherein a comparison phase vector is generated which is varied until it coincides with the actual phase vector from the rotary encoder with a desired accuracy.
A common drawback of these heretofore known methods is that, initially, angle information is produced, which can be used only via at least one further processing step for generating a clock signal. In principle, the angle information is made available only at discrete times, because of the sampling rate of the A/D converter. In order to use this angle information to generate a clock signal of a required frequency, in the case of the heretofore known methods it is necessary to obtain the angle information at a higher frequency than the clock signal. The formation of non-even-numbered interpolation factors is possible only to a restricted extent or with great outlay or expense.
It is therefore an object of the invention, to provide a clock generator for an imaging device, which, even under unfavorable operating conditions, ensures that high accuracy is achieved when setting an image on a printing form.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a clock generator for generating a clock signal for an imaging device serving for transferring image-setting information to a rotatable printing form, the clock generator comprising an input to which an actual value signal dependent upon an angular position of the printing form is applicable, and an output serving for outputting the clock signal, and further comprising a first comparator having a first input whereto the actual value signal is applicable as an analog signal, a second input to which an analog nominal value signal, respectively, representing a nominal value for the angular position of the printing form, is applicable, and an output serving for outputting the clock signal formed as a function of a result of a comparison between the analog actual value signal and the analog nominal value signal.
In accordance with another feature of the invention, the clock first comparator serves for outputting a clock pulse, respectively, when there is agreement between the amplitude of the analog actual value signal and the amplitude of the analog nominal value signal.
In accordance with a further feature of the invention, the clock generator further comprises a memory wherein the nominal values for the angular position of the printing form are stored.
In accordance with an added feature of the invention, the clock generator further comprises an arithmetic unit for determining the nominal values for the angular position of the printing form.
In accordance with an additional feature of the invention, the clock generator further comprises a correction value memory wherein correction values for correcting the nominal values for the angular position of the printing form are stored.
In accordance with yet another feature of the invention, the clock generator further comprises a digital/analog converter into which the nominal values for generating the analog nominal value signal are feedable.
In accordance with yet a further feature of the invention, the actual value signal has a sinusoidal waveform.
In accordance with yet an added feature of the invention, the clock generator further comprises a second comparator having an input to which the analog actual value signal for the angular position of the printing form is applicable, and the second comparator having an output serving for outputting a starting signal for starting the clock generator when the analog actual value signal satisfies a prescribed condition.
In accordance with another aspect of the invention, there is provided an imaging device for a printing machine, comprising a clock generator for generating a clock signal for the imaging device which serves for transferring image-setting information to a rotatable printing form, the clock generator comprising an input to which an actual value signal dependent upon an angular position of the printing form is applicable, and an output serving for outputting the clock signal, and further comprising a first comparator having a first input whereto the actual value signal is applicable as an analog signal, a second input to which an analog nominal value signal, respectively, representing a nominal value for the angular position of the printing form, is applicable, and an output serving for outputting the clock signal formed as a function of a result of a comparison between the analog actual value signal and the analog nominal value signal.
In accordance with a further aspect of the invention, there is provided a printing machine, comprising a clock generator for generating a clock signal for an imaging device serving for transferring image-setting information to a rotatable printing form, the clock generator comprising an input to which an actual value signal dependent upon an angular position of the printing form is applicable, and an output serving for outputting the clock signal, and further comprising a first comparator having a first input whereto the actual value signal is applicable as an analog signal, a second input to which an analog nominal value signal, respectively, representing a nominal value for the angular position of the printing form, is applicable, and an output serving for outputting the clock signal formed as a function of a result of a comparison between the analog actual value signal and the analog nominal value signal.
In accordance with an additional aspect of the invention, there is provided a method for setting an image on a rotatable printing form, which comprises transferring image-setting information to the printing form as a function of a clock signal depending upon an angular position of the printing form, after determining the clock signal from an analog actual value signal dependent upon the angular position of the printing form and an analog nominal value signal specifying the respective nominal value for the angular position of the printing form, via an analog comparison.
In accordance with another mode, the method of the invention further comprises determining the nominal values for the angular position of the printing form in advance in a calibration run, and storing the determined nominal values.
In accordance with a further mode, the method of the invention further comprises determining correction values for correcting the nominal values for the angular position of the printing form in advance in a calibration run, and storing the determined correction values.
In accordance with a concomitant mode, the method of the invention further comprises processing at least two actual value signals having a phase offset relative to one another.
In order to achieve great accuracy when setting an image on a rotatable printing form, the clock generator according to the invention outputs at the output thereof a highly accurate clock signal for the imaging device, the signal being synchronized exactly with the rotational movement of the printing form. Applied to the input of the clock generator is an actual value signal which depends upon the angular position of the printing form. As an essential component, the clock generator has a first comparator with a first input, to which the actual value signal is applied as an analog signal. A second input of the first comparator has an analog nominal value signal applied thereto which, respectively, represents a nominal value for the angular position of the printing form. At the output of the comparator, the clock signal formed as a function of the result of a comparison between the analog actual value signal and the analog nominal signal is outputted. This clock signal or a signal derived therefrom is ultimately used for controlling the imaging device, i.e., for determining the timing during the transmission of the imaging or image-setting information to the printing form.
The clock generator according to the invention offers the advantage that the clock signal output is synchronized exactly with the rotational movement of the printing form at every time, to be specific, even when this rotational movement is not uniform.
In a preferred embodiment of the clock generator according to the invention, the first comparator, respectively, when there is agreement between the amplitude of the analog actual alue signal and the amplitude of the analog nominal value signal, outputs a clock pulse, i.e., the clock pulse is outputted exactly when the printing form reaches the angular position prescribed by the analog nominal value signal.
The nominal values for the angular position of the printing form can be stored in a memory of the clock generator. As an alternative thereto, the clock generator can also have an arithmetic unit for determining the nominal values for the angular position of the printing form.
The clock generator according to the invention additionally offers the advantage that a correction value memory may be provided, wherein correction values for correcting the nominal values for the angular position of the printing form are stored. This makes it possible to take into account any deviations of the geometry of the printing form from a prescribed or predefined ideal shape, or other errors in the nominal values, and therefore to compensate therefor as early as during the generation of the clock signal, so that subsequent error correction, for example, during the control of the imaging device, is not required.
In addition, the clock generator according to the invention offers the advantage that it is able to process actual value signals with a sinusoidal waveform, as are generated by commercially available angular position transmitters (rotary encoders).
A starting signal for starting the clock generator according to the invention can be generated by a second comparator. The second comparator outputs the starting signal when the analog actual value signal applied to the input thereof satisfies a prescribed condition.
In the method according to the invention for setting an image on a rotatable printing form, imaging or image-setting information is transferred to the printing form as a function of the clock signal, which depends upon the angular position of the printing form. The clock signal is determined from the analog actual value signal, which depends upon the angular position of the printing form, and the analog nominal value signal, which specifies the respective nominal value for the angular position of the printing form, via an analog comparison.
The nominal values for the angular position of the printing form can be determined in advance in a calibration run, and stored. This offers the advantage that the nominal values are matched to the actual conditions, and error correction when carrying out the setting of an image is rendered superfluous.
As an alternative thereto, it is also possible to determine and to store correction values for correcting the nominal values for the angular position of the printing form during the calibration run carried out in advance. These correction values can be used both in an embodiment of the clock generator wherein the nominal values are read from the memory during the imaging operation, and in an embodiment wherein the nominal values are calculated.
In order to increase the accuracy further, in a modification of the method according to the invention, at least two analog actual value signals, which have a phase offset in relation to one another, are processed, for example, alternately. This offers the advantage that, at any time, an analog actual value signal is processed having a slope of sufficiently high magnitude, so that, respectively, very precise triggering of the clock pulses is possible.
In addition, besides at least one actual value signal, a so-called reference signal can be generated by the angular position transmitter and, if necessary or desirable, processed in the clock generator. A reference signal of this type can be outputted, for example, at a defined angular position of the angular position transmitter during each revolution. Evaluation or processing of this reference signal is particularly advantageous for corrections, because for this purpose information about the absolute angle of the angular position transmitter is helpful.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a clock generator for an imaging device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.